FK Com Research Articles

The First Photometric Analysis of Two Low-mass-ratio Contact Binary Systems in TESS Survey

Low-mass-ratio (q) contact binary systems are progenitors of stellar mergers such as blue stragglers or fast-rotating FK Com stars. In this study, we present the first light curve analysis of two newly identified low mass-ratio contact binary systems, TIC 55007847 and TIC 63597006, that are identified from TESS. Both stars are classified as A-subtype contact binaries. We obtained the precise orbit periods for the two objects by using the O − C method, i.e., P = 0.6117108 day for TIC 55007847 and P = 0.7008995 day for TIC 63597006, and found an obvious periodic signal in the O − C curve of TIC 63597006. We suggest that the periodic signal comes from a third body. We further use the Markov chain Monte Carlo method with PHOEBE to derive the photometric solutions for the two binaries. The photometric solution for this object shows that the contribution of the third body is about 6%. Our analysis revealed that TIC 55007847 has an extremely low mass ratio of q = 0.08. By calculating the ratio of spin angular momentum to the orbital angular momentum J s/J o, we found that TIC 55007847 is very close to the instability threshold with J s/J o = 0.31, indicating that it may merge into a single, fast-rotating star in the future. For TIC 63597006, q = 0.14 and J s/J o = 0.15. This object is in a relatively stable evolutionary status at present.

ACTIVITY CYCLES OF FK COM

New results of photometric observations of the chromospherically active star FK Com (a prototype of the group of the same name) performed during the past 5 years (2018–2023) at the INASAN observatories in Zvenigorod, Simeiz Observatory INASAN and Russian–Cuban Observatory in Havana, Republic of Cuba, are presented. In total during this observation interval we obtained 9060 estimates of the brightness of the star in V band. Our measurements as well as data from the literature and from the Kamogata Wide-field Survey (KWS) archive were combined into the array comprising 17 653 measurements in an interval of about 57 years. Based on the power spectrum constructed from these data, P values of the cycle activity have been established, which according to our estimate are equal to 2.4, 5.63, 8, 13.6, 30 and 49 years. The dominant is the P cycle lasting 5.63 years. It is shown that this dominant cycle with a period of about 5.63 years found by us according to more extensive data (5.4–5.8 years in other sources) can be traced in the results of the analysis of previous studies. The results on the activity cycles of FK Com are compared with data on the long-term variability of two more stars of the type under consideration – HD 199178 (V1794 Cyg) and ET Dra. Based on the data on the P cycle of other chromospherically active stars (according to literature sources and our measurements) a diagram \(\log (1{\text{/}}{{P}_{{{\text{rot}}}}}){\kern 1pt} - {\kern 1pt} \log ({{P}_{{{\text{cycl}}}}}{\text{/}}{{P}_{{{\text{rot}}}}})\) is analyzed. The conclusion about the comparability of the values of the activity cycles of the FK Com type stars with the data for stars of the RS CVn type is made.

ACTIVITY CYCLES OF SINGLE G5 III-IV GIANT HD 199178

A unique series of photometric data has been obtained for a period of more than a hundred years for the rapidly rotating single G5 III–IV giant HD 199178 (V1794 Cyg) belonging to the group of stars of the FK Com type. Our analysis of the long-term periodicity of the activity of this star is based on all available measurements of its brightness in B filter. To assess the brightness of HD 199178 in the era preceding photoelectric and CCD observations measurements were made using photographic plates from the archive of the SAI MSU at the Krasnopresnenskaya Observatory of Moscow State University in Moscow (4 plates taken from 1898 to 1903 and 41 plates obtained from 1935 to 1958). As a result, a total of 2142 estimates of the brightness of the star in the B filter were considered . They have a unique duration of 118.3 years and cover the time interval from 1898 to July 2016. Evidence of the existence of long-term cycles of photometric variability with values of the order of 25–60 years has been found. It is assumed that there are possible activity cycles of 2000, 3165, 5050, 9000 and 21600 days (5.5, 8.7, 16.6, 24.7 and 59.2 years). The results obtained are compared with other estimates of activity cycles of HD 199178. The existence of a cycle of 8.7–9 years should be recognized as the most reliable. It is found that to combine data into a single array the conversion of values B into values V using the average value of the color index (\(B{\kern 1pt} - {\kern 1pt} V\)) is not possible due to changes (including cyclical) of (\(B{\kern 1pt} - {\kern 1pt} V\)) with time.

Activity Cycles of a Single G5 III–IV Giant HD 199178

We obtained a unique series of photometric data for more than a hundred years for a rapidly rotating single G5 III–IV giant HD 199178 (V1794 Cyg), belonging to FK Com type stars. The analysis of the long-term variation of the activity of this star in the filter \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B$$\\end{document} is based on all measurements available in literary sources. To evaluate the brightness of HD 199178 in the era preceding photoelectric and CCD-observations, the measurements of photographic plates from the SAI MSU archive were shot at the Krasnopresnenskaya Observatory of Moscow State University in Moscow (4 records shot from 1898 to 1903, and 41 records for the period from 1935 to 1958) were performed. As a result, a total of 2142 assessments of the brightness of the star in the filter \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B$$\\end{document} were received. They have a unique duration of 118.3 yr and cover the time interval from 1898 to July 2016. Evidence was found of the existence of long-term cycles of photometric variability lasting for about 25‒60 years. We put forward an assumption on existence of possible activity cycles of 2000, 3165, 5050, 9000, and 21 600d (respectively, 5.5, 8.7, 16.6, 24.7, and 59.2 years). The results are compared with other estimates of activity cycles in HD 199178. The most reliable should be recognizing the existence of a cycle lasting 8.7‒9 years. It was found that in order to combine data into a single array, the convertation of \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B$$\\end{document} magnitudes into \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$V$$\\end{document} magnitudes using the average value of the color index (B–V) is not possible due to the changes (including cyclic) of color index (B–V) over time.

Спектральные наблюдения хромосферно-активной звезды FK Com

Представлены результаты новых спектральных наблюдений хромосферно-активной звезды FK Com в 2021 и 2022 г. в Симеизской обсерватории и в обсерватории Синлонг (Xinglong, NAOC, Китай). Рассмотрена эволюция профилей линии H α в спектре FK Com. Новые наблюдения показали, что зарегистрированные нами изменения формы профилей линии H α аналогичны опубликованным ранее в литературе. По нашим спектрограммам, полученным для фаз вращения звезды близким к 0.7 с интервалом около года (в 2021 и 2022 г.) и полученным на обсерваториях KPNO (в 2000 г.) и NOT в 1998, 1999, 2001, 2003, 2004 г. для этой же фазы, установлены различия в профилях линии H α . Вероятно, изменения профилей линии H α в большей степени зависят от уровня активности звезды, чем от фазы вращения. We present the results of new spectral observations of the chromospherically active star FK Com in 2021 and 2022 at the Simeiz Observatory and the Xinglong Observatory (NAOC, China). The evolution of the pro les of the H α line in the spectrum of FK Com is considered. New observations indicate that the changes in the shape of the pro les of the H α line registered by us are similar to those previously published in the literature. According to our spectra obtained for the phases of rotation of the star close to 0.7 with an interval of about a year (in 2021 and 2022) and obtained at the KPNO (2000) and NOT observatories in 1998, 1999, 2001, 2003, 2004 for the same phase, di erences in the pro les of the H α line are established. Probably, the changes in the pro les of the H α line depend more on the level of activity of the star than on the phase of rotation.

Activity of ET Dra—a Star of FK Com Type

Activity of ET Dra—a Star of FK Com Type

KIC 2142183 as a FK Com Star Candidate

KIC 2142183 as a FK Com Star Candidate

X-Ray Emission from Candidate Stellar Merger Remnant TYC 2597-735-1 and Its Blue Ring Nebula

Abstract Tight binary or multiple-star systems can interact through mass transfer and follow vastly different evolutionary pathways than single stars. The star TYC 2597-735-1 is a candidate for a recent stellar merger remnant resulting from a coalescence of a low-mass companion with a primary star a few thousand years ago. This violent event is evident in a conical outflow (“Blue Ring Nebula”) emitting in UV light and surrounded by leading shock filaments observed in Hα and UV emission. From Chandra data, we report the detection of X-ray emission from the location of TYC 2597-735-1 with a luminosity log ( L X / L bol ) = − 5.5 . Together with a previously reported period of ~14 days, this indicates ongoing stellar activity and the presence of strong magnetic fields on TYC 2597-735-1. Supported by stellar evolution models of merger remnants, we interpret the inferred stellar magnetic field as dynamo action associated with a newly formed convection zone in the atmosphere of TYC 2597-735-1, though internal shocks at the base of an accretion-powered jet cannot be ruled out. We speculate that this object will evolve into an FK Com–type source, i.e., a class of rapidly spinning magnetically active stars for which a merger origin has been proposed but for which no relic accretion or large-scale nebula remains visible. We also detect likely X-ray emission from two small regions close to the outer shock fronts in the Blue Ring Nebula, which may arise from inhomogeneities either in the circumstellar medium or in the mass and velocity distribution in the merger-driven outflow.

Spectropolarimetric follow-up of 8 rapidly rotating, X-ray bright FK Comae candidates

ABSTRACT Our understanding of the evolved, rapidly rotating, magnetically active, and apparently single FK Comae stars is significantly hindered by their extreme rarity: only two stars in addition to FK Com itself are currently considered to be members of this class. Recently, a sample of more than 20 candidate FK Comae type stars was identified within the context of the Kepler–Swift Active Galaxies and Stars (KSwAGS) survey. We present an analysis of high-resolution Stokes V observations obtained using ESPaDOnS@CFHT for 8 of these candidates. We found that none of these targets can be considered members of the FK Comae class based primarily on their inferred rotational velocities and on the detection of spectroscopic binary companions. However, 2 targets show evidence of magnetic activity and have anomalously high projected rotational velocities (vsin i) relative to typical values associated with stars of similar evolutionary states. EPIC 210426551 has a $v\sin {i}=209\, {\rm km\, s}^{-1}$, an estimated mass of $1.07\, \mathrm{ M}_\odot$, and, based in part on its derived metallicity of [M/H] = −0.4, it is either an evolved main sequence (MS) star or a pre-MS star. KIC 7732964 has a mass of $0.84\, \mathrm{ M}_\odot$, lies near the base of the red giant branch, and exhibits a $v\sin {i}=23\, {\rm km\, s}^{-1}$. We find that these two objects have similar characteristics to FK Com (albeit less extreme) and that their rapid rotation may be inconsistent with that predicted for a single star evolutionary history. Additional observations are necessary in order to better constrain their evolutionary states and whether they have short-period binary companions.

Activity of the Star ET Dra

V-band photometric observations of the FK Com type star ET Dra were made at the Zvenigorod observatory (INASAN) during March-April 2018. The new data were used in a solution of the inverse problem of recovering the surface temperature inhomogeneities for ET Dra to study the position of cold regions on its surface which were compared with photometric observations made in 1990.

A search for FK Com candidates using Kepler Space Telescope observations: Analogs of HD 199178

Analysis of collected photometric observations obtained with the Kepler Space Telescope were used to select and study 33 objects with parameters corresponding to those of the FK Com starHD199178; these can be considered candidate stars of this type. In this final study, the four objects with the best light curves, which show the properties of their regular rotational modulation most clearly, were selected for detailed studies. The photometric analysis is based on all data currently available in the Kepler archive (covering almost four years). The rotational periods and estimated parameters of the objects’ differential rotation are determined, and the longitudes of the dominant active regions on the surfaces found. For all four stars, the spot coverage is approximately 1% of the visible stellar surface area. The rotational periods and data on the stars’masses and radii fromtheMAST catalog are used to determine the rotation velocities projected onto the line of sight, which ranged from 12 to 21 km/s. Further studies will enable definite conclusions about how these stars are related to FK Com stars. If they are ultimately classified as FK Com stars, this will considerably increase the number of this rare type of star and the also number of rapidly rotating, single, late-type giants.

RAPIDLY ROTATING, X-RAY BRIGHT STARS IN THEKEPLERFIELD

We present Kepler light curves and optical spectroscopy of twenty X-ray bright stars located in the Kepler field of view. The stars, spectral type FK, show evidence for rapid rotation including chromospheric activity 100 times or more above the Sun at maximum and flaring behavior in their light curves. Eighteen of our objects appear to be (sub)giants and may belong to the class of FK Com variables, that is evolved rapidly spinning single stars with no excretion disk and high levels of chromospheric activity. Such stars are rare and are likely the result of W UMa binary mergers, a process believed to produce the FK Com class of variable and their descendants. The FK Com stage, including the presence of an excretion disk, is short-lived but leads to longer-lived stages consisting of single, rapidly rotating evolved (sub)giants with high levels of stellar activity.

Detecting abrupt changes in the spectra of high-energy astrophysical sources

Variable-intensity astronomical sources are the result of complex and often extreme physical processes. Abrupt changes in source intensity are typically accompanied by equally sudden spectral shifts, that is, sudden changes in the wavelength distribution of the emission. This article develops a method for modeling photon counts collected from observation of such sources. We embed change points into a marked Poisson process, where photon wavelengths are regarded as marks and both the Poisson intensity parameter and the distribution of the marks are allowed to change. To the best of our knowledge, this is the first effort to embed change points into a marked Poisson process. Between the change points, the spectrum is modeled nonparametrically using a mixture of a smooth radial basis expansion and a number of local deviations from the smooth term representing spectral emission lines. Because the model is over-parameterized, we employ an $\ell_{1}$ penalty. The tuning parameter in the penalty and the number of change points are determined via the minimum description length principle. Our method is validated via a series of simulation studies and its practical utility is illustrated in the analysis of the ultra-fast rotating yellow giant star known as FK Com.

FK COMAE BERENICES, KING OF SPIN: THE COCOA-PUFS PROJECT* †

ABSTRACT COCOA-PUFS is an energy-diverse, time-domain study of the ultra-fast spinning, heavily spotted, yellow giant FK Comae Berenices (FK Com: HD117555; G4 III). This single star is thought to be a recent binary merger, and is exceptionally active by measure of its intense ultraviolet (UV) and X-ray emissions, and proclivity to flare. COCOA-PUFS was carried out with the Hubble Space Telescope in the UV (1200–3000 Å), using mainly its high-performance Cosmic Origins Spectrograph, but also high precision Space Telescope Imaging Spectrograph; Chandra X-ray Observatory in the soft X-rays (0.5–10 keV), utilizing its High-Energy Transmission Grating Spectrometer; together with supporting photometry and spectropolarimetry in the visible from the ground. This is an introductory report on the project. FK Com displayed variability on a wide range of timescales over all wavelengths during the week-long main campaign, including a large X-ray flare; “super-rotational broadening” of the far-ultraviolet “hot lines” (e.g., Si iv 1393 Å; 8 × 104 K) together with chromospheric Mg ii 2800 Å and C ii 1335 Å (1–3 × 104 K); large Doppler swings suggestive of bright regions alternately on advancing and retreating limbs of the star; and substantial redshifts of the epoch-average emission profiles. These behaviors paint a picture of a highly extended, dynamic, hot (∼10 MK) coronal magnetosphere around the star, threaded by cooler structures perhaps analogous to solar prominences and replenished continually by surface activity and flares. Suppression of angular momentum loss by the confining magnetosphere could temporarily postpone the inevitable stellar spindown, thereby lengthening this highly volatile stage of coronal evolution.

Possibilities for identifying FK com candidates using observations with the Kepler Space Telescope

High-accuracy photometric observations obtained with the Kepler Space Telescope are used to identify candidate FK Com stars-a very rare group of single, rapidly rotating, chromospherically active G-K stars. Published data for more than 40 000 stars are used with available Kepler observations from the Q3 time interval to select four stars with temperature ranges, surface gravities, and rotation periods consistent with those of FK Com stars. These stars also display brightness variations with considerable amplitudes, possibly testifying to the presence of appreciably spotting on their surfaces. The rotation periods of these stars are determined, and the parameters of their differential rotation estimated. The locations (longitudes) of the dominant active regions on the stellar surfaces are identified. In all cases, the active longitude does not remain constant, andmoves across the stellar surface with time. In general, the character of this activeregion movement is the same as that found earlier for FK Com and HD 199178. These displacements are characterized by monotonic motions over hundreds of days, as well as changes in the positions by about 180° (“flip-flops”) or phase shifts not exceeding 0.4 in phase. The number of active-longitude position changes during the studied time interval ranges from one for KIC 11862915 to seven for KIC 5785906 (five phase shifts are also detected for the latter star). The time scale for the position changes of the active longitudes is from 1500 days (about 4 years) to 200 days (0.54 years), comparable to the reported time intervals between flip-flops for FK Com (from 0.8 to 4.4 years). The duration of the stellar activity cycles are estimated by analyzing the amplitude spectrum for changes in the brightness-variation amplitudes for datasets covering a single rotation period. The photometric variations of the stars on various time scales (from the rotation period, which reveals the presence of surface temperature inhomogeneities, to activity cycles lasting for several years) are similar to those derived for FK Com and other stars of this type. The need for spectroscopic observations of the selected candidates to establish whether they are single (do not show signs of binarity), look for emission lines of chromospheric origin, estimate the lithium abundances, and determine the stellar rotation velocities from spectral-line profiles is noted.

A study of the magnetic field in FK Com

We present the results of our new spectropolarimetric observations of FKCom aimed to measure the longitudinal component Bz of its magnetic field. The most interesting interpretation of our results suggests that the Bz value has significantly decreased compared to the 2008 observations of this star. Such a decrease of the longitudinal component of the magnetic field can be similar to the secular variations of Bz registered earlier for another chromospherically active star II Peg. On the other hand, assuming the existence of Bz variations with the rotation phase, we suggest that the variations of the phase curve Bz from 2008 to 2012 originated because of the strengthening of the negative polarity spot, its domination, and as a result-a generally more symmetric distribution of magnetic regions.

Flip-flops of FK Comae Berenices

FK Comae is a rapidly rotating magnetically active star, the light curve of which is modulated by cool spots on its surface. It was the first star where the "flip-flop" phenomenon was discovered. Since then, flip-flops in the spot activity have been reported in many other stars. Therefore, it is of interest to perform a more thorough study of the evolution of the spot activity in FK Com. In this study, we analyse 15 years of photometric observations with two different time series analysis methods, with a special emphasis on detecting flip-flop type events from the data. We apply the continuous period search and carrier fit methods on long-term standard Johnson-Cousins V-observations from the years 1995--2010. The observations were carried out with two automated photometric telescopes, Phoenix-10 and Amadeus T7 located in Arizona. We identify complex phase behaviour in 6 of the 15 analysed data segments. We identify five flip-flop events and two cases of phase jumps, where the phase shift is \Delta \phi < 0.4. In addition we see two mergers of spot regions and two cases where the apparent phase shifts are caused by spot regions drifting with respect to each other. Furthermore we detect variations in the rotation period corresponding to a differential rotation coefficient of |k|>0.031. The flip-flop cannot be interpreted as a single phenomenon, where the main activity jumps from one active longitude to another. In some of our cases the phase shifts can be explained by differential rotation: Two spot regions move with different angular velocity and even pass each other. Comparison between the methods show that the carrier fit utility is better in retrieving slow evolution especially from a low amplitude light curve, while the continuous period search is more sensitive in case of rapid changes.

K 1-6: An Asymmetric Planetary Nebula with a Binary Central Star

AbstractWe present new imaging data and archival multiwavelength observations of the little-studied emission nebula K 1-6 and its central star. Narrow-band images inHα (+[N II]) and [O III] taken with the Faulkes Telescope North reveal a stratified, asymmetric, elliptical nebula surrounding a central star which has the colours of a late G or early K-type subgiant or giant. GALEX ultraviolet images reveal a very hot subdwarf or white dwarf coincident in position with this star. The cooler, optically dominant star is strongly variable with a period of 21.312± 0.008 days, and is possibly a high-amplitude member of the RS CVn class, although an FK Com classification is also possible. Archival ROSAT data provide good evidence that the cool star has an active corona. We conclude that K 1-6 is most likely an old bona fide planetary nebula at a distance of ∼1.0 kpc, interacting with the interstellar medium, and containing a binary or ternary central star. The observations and data analyses reported in this paper were conducted in conjunction with Year 11 high school students as part of an Australian Research Council Linkage Grant science education project, denoted Space To Grow, conducted jointly by professional astronomers, educational researchers, teachers, and high-school students.

MAGNETIC STRUCTURE OF RAPIDLY ROTATING FK COMAE-TYPE CORONAE

We present a three-dimensional simulation of the corona of an FK Com-type rapidly rotating G giant using a magnetohydrodynamic model that was originally developed for the solar corona in order to capture the more realistic, non-potential coronal structure. We drive the simulation with surface maps for the radial magnetic field obtained from a stellar dynamo model of the FK Com system. This enables us to obtain the coronal structure for different field topologies representing different periods of time. We find that the corona of such an FK Com-like star, including the large scale coronal loops, is dominated by a strong toroidal component of the magnetic field. This is a result of part of the field being dragged by the radial outflow, while the other part remains attached to the rapidly rotating stellar surface. This tangling of the magnetic field,in addition to a reduction in the radial flow component, leads to a flattening of the gas density profile with distance in the inner part of the corona. The three-dimensional simulation provides a global view of the coronal structure. Some aspects of the results, such as the toroidal wrapping of the magnetic field, should also be applicable to coronae on fast rotators in general, which our study shows can be considerably different from the well-studied and well-observed solar corona. Studying the global structure of such coronae should also lead to a better understanding of their related stellar processes, such as flares and coronal mass ejections, and in particular, should lead to an improved understanding of mass and angular momentum loss from such systems.

Spots and activity cycles of HD 199178

We present our analysis of photometry for the FK Com star HD 199178 (V1794 Cyg). The V-band light curves are used to restore the distribution of temperature inhomogeneities on the stellar surface. The spots on the surface of HD 199178 are concentrated at two preferred longitudes separated by 0.5 in phase (180° in longitude). In addition to the quasi-periodic switching of the most active area between these two longitudes, which occurs in cycles of 2.1–2.4 or 4.1 years, we suspect that the two active areas moved toward each other across the stellar surface, possibly merging into a single formation. The detected cycle in the star’s brightness variations of about 8.0 years is also clearly visible in variations of the star’s spottedness. Themagnetic activity of the FK Com star HD199178 is in many ways similar to that observed for the prototype of this group.